The present invention relates to a method of fabricating a thin film semiconductor device in which thin film transistors each having an active layer made from polycrystalline silicon are integrally formed on a substrate, and more particularly to a technology for fabricating a polycrystalline silicon thin film transistor by a low temperature process.
A thin film semiconductor device, which is typically suitable as a drive substrate for a display such as an active matrix type liquid crystal display, is being extensively developed at present. An active layer of a thin film transistor is made from polycrystalline silicon or amorphous silicon. In particular, attention has been focused on a technology regarding a polycrystalline silicon thin film transistor capable of realizing a high precision active matrix type color liquid crystal display with a small-sized structure. To be more specific, in this technology, a polycrystalline silicon thin film, which has been used only as an electrode material or a resistance material in the related art semiconductor technology, is utilized for an active layer of each of thin film transistors used as pixel switching elements formed on an insulating substrate made from transparent glass or the like. The technology using a polycrystalline silicon thin film transistor seems to be the only one means capable of realizing thin film transistors used for high performance switching elements enabling a high density design for ensuring such an image quality as to meet the needs of the market. Such a technology simultaneously allows a peripheral circuit section having been composed of external ICs to be formed on the same substrate together with a pixel array section by the same process. The technology using a polycrystalline silicon thin film is thus allowed to realize a high precision active matrix liquid crystal display of a type integrated with a peripheral circuit section which has been never allowed to be realized by the technology using an amorphous silicon thin film transistor.
Since polycrystalline silicon exhibits a carrier mobility larger than that of amorphous silicon, a current drive ability of a polycrystalline silicon thin film transistor becomes higher. As a result, a peripheral circuit section requiring high speed driving such as a horizontal scanning circuit and a vertical scanning circuit can be formed on the same substrate simultaneously with thin film transistors as pixel switching elements. This makes it possible to significantly reduce the number of signal lines taken out of a thin film semiconductor device as a display. Further, a CMOS circuit in which N-channel type and P-channel type thin film transistors are integrally formed can be of configured as an on-chip structure. This enables integration of a level shift circuit, thus realizing low voltage driving of a timing signal.
As a device technology and a process technology regarding a thin film transistor, there has been already established a high temperature process technology adopting a processing temperature of 900.degree. C. or more. A feature of the high temperature process lies in reforming a semiconductor thin film formed on a high heat-resisting substrate made from quartz or the like by a solid-phase growth process. In the solid-phase growth process, a semiconductor thin film is processed by heat-treatment at a temperature of 600.degree. C. or more. At this heat-treatment, individual crystal grains contained in a polycrystalline silicon body composed of an aggregation of fine silicon crystals at the film formation stage becomes larger. A polycrystalline silicon film obtained by the solid-phase growth process and high-temperature heat treatment thereafter exhibits a high carrier mobility of about 100 cm.sup.2 /v.times.s. To carry out the above high temperature process, it is essential to adopt a high heat-resistant. As such a substrate, there has been used an expensive material such as quartz. It is apparent that quartz is disadvantageous in terms of reduction in fabrication cost.
In place of the above-described high temperature process, there has been developed a low temperature process adopting a processing temperature of about 600.degree. C. or less. As one means for realizing a process of fabricating a thin film semiconductor device by the low temperature process, laser annealing using a laser beam has been examined. The laser annealing process involves forming a non-single crystal semiconductor thin film such as an amorphous or a polycrystalline silicon thin film on a low heat-resisting insulating substrate made from glass or the like, irradiating a laser beam to the semiconductor thin film to locally heat and fuse it, and crystallizing the semiconductor thin film at the cooling step. The semiconductor thin film thus crystallized is used as an active layer (channel region) of each of polycrystalline silicon thin film transistors which are integrally formed on the substrate. Since the crystallized semiconductor thin film exhibits a high carrier mobility as described above, it is possible to integrally form high performance thin film transistors by processing the crystallized semiconductor thin film.
In a liquid crystal display of a type containing a peripheral circuit (monolithic type) using an insulating substrate made from glass or the like, a polycrystalline silicon thin film transistor is required to have a good transistor characteristic, for example, a uniform and high drive current, for displaying a uniform image. That is, the silicon thin film must be reformed to exhibit a high carrier mobility. To be more specific, the performance of a thin film transistor must be improved by, as described above, crystallizing an amorphous silicon thin film. In this case, at present, to crystallize an amorphous silicon thin film, it is expected to adopt laser annealing using an excimer laser light source. As a manner of irradiating excimer laser light, there is being extensively developed a technology in which an excimer laser pulse having a linear or rectangular shaped uniform energy density is subjected to multi-scan shot. In general, as a process of forming an amorphous silicon film, there has been adopted a chemical vapor deposition (CVD) process, particularly, a PE (Plasma Enhanced) CVD process capable of depositing amorphous silicon on a glass sheet having a large area at a low temperature with a good throughput. In this CVD process, since a hydride is used as a source gas, hydrogen is contained in a silicon thin film in a large amount which reaches about 10 to 20 at %. Accordingly, if the amorphous silicon thin film containing a large amount of hydrogen is directly crystallized by laser annealing, hydrogen in the film tends to be aggregated to cause voids, which is inconvenient for fabrication of the thin film semiconductor device. For this reason, in general, before laser annealing for crystallization, the amorphous silicon film containing a large amount of hydrogen is subjected to a preliminary annealing in a heating furnace for removal of hydrogen from the film (dehydrogenation). However, even after such a pre-treatment, hydrogen remains in the film in an amount of about several at %, as a result of which there occur fine voids upon rapid heating by laser annealing, tending to locally deteriorate the silicon thin film. Also, in the PECVD process, PH.sub.3 or B.sub.2 H.sub.6 in a vapor-phase is occasionally mixed for introducing an impurity in a silicon thin film; however, due to adhesion of such a material on a tube wall of a film formation chamber and/or a limitation of controllability of mass flow, it is generally difficult to introduce an impurity in the entire silicon thin film at a relatively low concentration at a high uniformity and a good repeatability.